Components for aerospace, automotive and like service applications have been subjected to the ever increasing demand for improvement in one or more mechanical properties while at the same time maintaining or reducing the weight of the component. To this end, the Charbonnier et al. U.S. Pat. No. 4 889 177 describes a method of making a composite casting wherein a molten lightweight alloy, such as magnesium or aluminum, is countergravity cast into a gas permeable sand mold having a fibrous insert of high strength ceramic fibers positioned therein by metallic seats so as to be incorporated into the casting upon solidification of the molten alloy.
The Funatani et al. U.S. Pat. No. 4 572 270 describes a method of making a composite casting to this same end wherein a mass of high strength reinforcing fibers, such as ceramic fibers, whiskers, or powder is incorporated into a lightweight metal matrix (e.g. aluminum or magnesium) that is die cast around the reinforcing mass in a pressure chamber.
A technique commonly referred to as bicasting has been employed in attempts to improve one or more mechanical properties of superalloy castings for use as aerospace components. Bicasting involves pouring molten metal into a mold cavity in which a preformed insert is positioned in a manner to augment one or more mechanical properties in a particular direction(s). The molten metal surrounds the insert and, upon solidification, yields a selectively reinforced casting comprising the insert embedded in and hopefully soundly bonded with the cast metal without contamination therebetween. However, as described in U.S. Pat. No. 4 008 052 attempts at practicing the bicasting process have experienced difficulty in consistently achieving a sound metallurgical bond between the insert and the metal cast therearound without bond contamination. Moreover, difficulty has been experienced in positioning the insert in the mold cavity and thus the final composite casting within required tolerances. The inability to achieve on a reliable and reproducible basis a sound, contamination-free bond between the insert and the cast metal has significantly limited use of bicast components in applications, such as aerospace components, where reliability of the component in service is paramount.
When a fiber reinforced metal matrix composite is used as the preformed insert in the bicasting process, reinforcing fibers exposed by machining the insert can react with the metal matrix during the transient thermal exposure imposed by bicasting. These reactions can adversely affect the reinforcing capabilities of the insert in the final bicast product.
It is an object of the present invention to provide an improved bicasting type of process for making a composite casting reinforced by a reinforcement insert, such as a fiber reinforced metal matrix composite insert or intermetallic reinforcement insert (e.g. a titanium aluminide insert), wherein a sound, void-free metallurgical bond is reliably and reproducibly produced between the reinforcement insert and the cast metal and wherein adverse reactions between the insert and the molten metal and between any exposed insert fibers and the insert matrix are reduced or eliminated.